The development potential of deep-ultradeep coalbed methane resources in the Longtan formation of the upper permian in the Sichuan Basin

The breakthrough in high-yield gas flow in the 1000-2500 m deep coalbed gas wells in the Junggar Basin, the DanningJixian and Yanchuan South blocks signifies the enormous potential for deep coal-bed gas development. The main buried depth of the thin to medium thick coal seam group (7-15 layers) in the Longtan Formation of the Upper Permian in the Sichuan Basin is 2000-4500 meters. In the central Sichuan region, there is a northward dipping monocline with locally developed low amplitude uplifts. The key development parameters such as coal seam gas content, gas saturation, and reservoir pressure are all better than those in the shallow part, which has strong potential to support the national natural gas development strategy of "increasing reserves and production". (1) The Longtan Formation coal seam has frequent coal mud interbedding, multiple coal seams, and a large cumulative resource. The lateral development of the 19 # coal seam is stable (average 3.2 m; >4 m favorable area of 700 km²). (2) The degree of thermal evolution of coal rock is high (2.53% -3.18%), with strong hydrocarbon generation ability. The total hydrocarbon content of coal measure gas shows good results, with a measured gas content of 16.64-17.61 m³/t at a depth of 2500 m, a gas saturation of 138% -151%, and free gas of about 4.84-5.6 m³/t. (3) The measured reservoir pressure coefficient at a depth of 800 m in southern Sichuan is greater than 1.1, and the predicted reservoir pressure coefficient in the central Sichuan area is greater than 1.8, indicating a potential abnormal ultrahigh pressure reservoir. Based on the resource endowment characteristics of coal seams in the Sichuan Basin, such as deep-ultra deep, overpressure, and supersaturation, and drawing on the successful experience of deep coalbed methane in the Daji area of eastern Hubei, taking the 19 # coal seam as the risk exploration target, implementing resource potential and gas reservoir characteristics, and exploring the construction of a three-dimensional exploration and development technology system for deep-ultra deep thin coalbed methane, it is of great significance for the large-scale production of - coalbed methane.